Amplifier circuit, an optical communication system and a method of controlling amplification

ABSTRACT

The invention concerns an amplifier circuit for receiving an optical signal. The circuit comprises a light sensitive member ( 301 ) which delivers an electric signal in response to an optical signal. Furthermore, the amplifier circuit comprises an amplifier unit ( 302 ) with a first input ( 304 ) which receives an electric signal from the light sensitive member ( 301 ). The amplifier unit ( 302 ) is of the kind which does not have any special input intended for controlling the amplification. A filter unit ( 310 ) prevents a possible direct current from the light sensitive member ( 301 ) from reaching the first input ( 304 ). A control unit ( 314 ) is connected to the first input ( 304 ) for controlling the amplification of the circuit. The invention also concerns an optical communication system comprising an amplifier circuit of the above-described kind. The invention also concerns a method of controlling the amplification in an optical input stage.

BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention concerns an amplifier circuit for receiving anoptical signal, which amplifier circuit comprises:

-   -   a light sensitive member arranged to receive an optical input        signal and to deliver an electric signal in response to the        received optical signal, and    -   an amplifier unit with a first input arranged to receive the        electric signal from the light sensitive member, wherein the        amplifier unit is arranged to influence the amplification of the        electric signal and to deliver an amplified output signal via a        first output, wherein the amplifier unit does not have any        further input specially intended for controlling the        amplification of the amplifier unit.

The invention also concerns a communication system and a method ofcontrolling the amplification in an optical input stage.

The above described amplifier circuit may suitably be used foramplifying an input signal originating from an optical fibre. Theamplifier circuit may thus constitute a fibre optic input stage. Such aninput stage is used in optical communication systems. Such an inputstage may be followed by further amplifiers. The input stage thus oftenensures that an input signal from an optical communication path isadapted to an input of a subsequent amplifier.

FIG. 1 shows schematically an example of such an optical input stageaccording to the prior art. The input stage comprises a light sensitivemember 301. The light sensitive member 301 may for example constitute aphoto-diode. The light sensitive member 301 delivers an electric signalin response to an optical input signal, for example from an opticalfibre (not shown in the figure). According to the shown example, thecathode of the photo-diode 301 is connected to a bias voltage V1. Thecircuit comprises an amplifier unit 302, which is often calledpreamplifier. The amplifier unit 302 has a first input 304 whichreceives an electric signal from the light sensitive member 301. Theamplifier unit 302 influences the amplification of the electric signaland delivers an amplified output signal via a first output 306. Theshown photo-diode 301 delivers a current into the input 304, wherein thestrength of the current depends on detected light. The amplifier unit302 converts the current to a voltage at the output 306. The transferfunction therefore gets the unit V/A, i.e. ohm. The amplification of theamplifier unit 302 may thus be stated in ohm. The amplifier unit 302 mayalso comprise an internal amplification control unit 308 which forexample may be arranged to reduce the amplification at too highcurrents.

The English abstract of JP-A-10284955 shows an example of this kind ofoptical input stage. This document shows such an input stage with acontrol circuit for controlling the amplification in response to anaverage value of the optical input power.

Also WO99/28768 shows an optical input stage where a control circuitcontrols a variable impedance element in the form of a diode connectedto an amplifier input.

The English abstract of JP-A-09298426 shows an optical input stage witha preamplifier. In this case the preamplifier has a special input wherea control signal may be connected for controlling the amplification ofthe preamplifier. However, an optical input stage usually lacks aspecial input for controlling the amplification. An example of an inputstage is the one which is sold with the name MC2006 of the fabricationMicrocosm. For example this input stage comprises an internal controlunit for reducing the amplification at too high currents. However, theinput stage lacks a special input for being able to control theamplification.

SUMMARY OF THE INVENTION

A purpose of the present invention is to achieve an amplifier circuitwhich makes it possible to control the amplification in an optical inputstage with an amplifier unit which does not have any special inputintended for controlling the amplification. Another purpose is toachieve such an amplifier circuit by relatively simple means.

These purposes are achieved by an amplifier circuit according to thefirst paragraph above and which is characterised by a filter unitarranged to prevent a possible direct current in the electric signalfrom the light sensitive member from reaching said first input, and acontrol unit connected to said first input and arranged to control thepower of the electric signal at this first input for thereby influencingthe power of said output signal.

The filter unit may for example constitute a capacitor. This filter unitdisconnects a possible direct current from the light sensitive member.Instead the control unit is connected to the first input. The power ofthe input signal at the first input is thus controlled with the help ofthe control unit instead of with the help of a direct current from thelight sensitive member. Thereby, the power of the output signal from thesecond output may be influenced with the help of the control unit, i.e.the purpose to be able to control the amplification of the amplifierunit is achieved.

According to an embodiment of the invention, said light sensitive memberis arranged such that a possible direct current from the light sensitivemember would be directed into said amplifier unit via the first input ifsaid filter unit were not arranged to prevent such a direct current,wherein the control unit is arranged to control the power of an electriccurrent in the direction in via the first input. According to thisembodiment, a light sensitive member may thus be used where the currentinto the first input depends on detected light intensity.

According to a further embodiment, the control unit is arranged as acontrollable current generator in that a variable amplificationcontrolling voltage unit is connected to a first resistance connected tosaid first input. This embodiment constitutes an advantageous and simpleexample of how the current into the first input may be controlled.

According to still an embodiment, said light sensitive member comprisesa first diode-unit with a first connection connected to a bias voltageand a second connection connected to said filter unit, which filter unitis connected to the first input. According to this embodiment, forexample a photo-diode of the kind which has been described above maythus be used as light sensitive member.

According to still an embodiment, the first connection constitutes thecathode side of the first diode unit and the second connectionconstitutes the anode side of the first diode unit. A photo-diodeusually functions such that its reverse current depends on incidentlight intensity. According to this embodiment, the current in towardsthe first input is thus controlled with the help of the diode unit.

According to a further embodiment, the amplifier circuit comprises asecond resistance connected to said second connection for conductingaway a possible direct current through the first diode unit. Since adirect current from the light sensitive member is prevented fromreaching the first input, this current must be conducted away in asuitable manner. A simple construction is to connect the light sensitivemember to a resistance which conducts this current away.

According to still an embodiment, the amplifier circuit comprises anamplification limiting unit connected to said first input. Hereby isprevented that the amplifier unit is set at a too high amplification.

According to another embodiment, the amplification limiting unitcomprises a third resistance connected between a voltage source and saidfirst input. If the amplifier unit is of the kind where a lower currentat the first input means a higher amplification, then suitably with thehelp of such a third resistance it may be ensured that always a certaincurrent is present at said first input. Thus a limitation of theamplification has been achieved in a simple manner.

According to another embodiment, said amplifier unit comprises aninternal amplification controlling unit, for example arranged forreducing the amplification at too high currents. As has been mentionedabove, the amplifier unit may also have an internal amplificationcontrol.

According to a further embodiment, a second diode unit is connected tosaid control unit for preventing incorrect current direction to/from thefirst input. For example if the amplifier circuit is arranged such thatalways a certain current is input via said first input, then the seconddiode unit may be arranged for preventing a current in the oppositedirection.

According to still an embodiment, the amplifier circuit comprises afilter member connected to said control unit for filtering out possibledisturbances in the electric signal which is controlled by the controlunit. It is hereby ensured that possible disturbances do not reach thefirst input.

According to a further embodiment, the amplifier circuit comprises afourth resistance with a first side connected to said light sensitivemember and a second side arranged to be connectable to a measuring unitfor measuring the current through the light sensitive member. It may bedesirable to be able to measure the photocurrent through the lightsensitive member, for example for determining the optical input power.In order to prevent that the measuring unit is connected to a pointwhich is sensitive to disturbances, suitably such a fourth resistancemay be arranged.

Another purpose of the invention is to achieve an optical communicationsystem. This purpose is achieved by an optical communication systemcomprising a first-transmitter-receiver unit, a secondtransmitter-receiver unit and at least one optical conduction memberwhich connects the first and the second transmitter-receiver units,wherein at least one of said transmitter-receiver units comprises anamplifier circuit according to any of the preceding embodiments. Such anoptical communication system has the advantage that the amplification ofthe input stage may be controlled according to the above-describedembodiments.

As has been mentioned above, another purpose of the invention is tooffer a method for controlling the amplification in an optical inputstage which comprises a light sensitive member arranged to receive anoptical input signal and to deliver an electric signal in response tothe received optical signal, and an amplifier unit with a first inputarranged to receive the electric signal from the light sensitive member,wherein the amplifier unit is arranged to influence the amplification ofthe electric signal and to deliver an amplified output signal via afirst output, wherein the amplifier unit does not have any further inputspecially intended for controlling the amplification of the amplifierunit.

This purpose is achieved by the following method steps: that a filterunit is arranged to prevent a possible direct current in the electricsignal from the light sensitive member from reaching said first input,and that a control unit is connected to said first input and arranged tocontrol the power of the electric signal at this first input for therebyinfluencing the power of said output signal. This method has the sameadvantages as have been described above in connection with the amplifiercircuit according to the invention.

Advantageous embodiments of the method are clear from the otherdependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be explained with the help of embodimentsand with reference to the annexed drawings.

FIG. 1 shows schematically an optical input stage according to the priorart.

FIG. 2 shows schematically a simple embodiment of the present invention.

FIG. 3 shows another embodiment of the present invention.

FIG. 4 shows schematically an optical communication system according tothe invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 2 shows an amplifier circuit according to the invention. Theamplifier circuit comprises a light sensitive member 301, for example aphoto-diode. In the shown example, the cathode of the photo-diode isconnected to a bias voltage V1. The anode of the photo-diode 301 isconnected to a filter unit 310, in this case a capacitor, which in itsturn is connected to a first input 304 of the amplifier unit 302. Thecapacitor 310 prevents a direct current from the photo-diode 301 fromreaching the first input 304. The hatched line 312 symbolises that sucha direct current is conducted away from the photo-diode 301. Theamplifier unit 302 has a first output 306 where an amplified outputsignal is delivered. The amplifier unit 302 is of the kind which doesnot have any special input intended for controlling the amplification ofthe amplifier unit 302. Such an amplifier unit 302 may suitablyconstitute a preamplifier component of the kind which has been describedinitially above. Such a component 302 may comprise an internalamplification controlling unit 308. Such an amplification controllingunit 308 may, but does not have to, comprise a feedback control loop.

According to the invention, a control unit 314 is connected to the firstinput. According to the shown example, the control unit 314 constitutesa variable current generator. With this current generator 314 thecurrent into the first input 304 may be controlled. The currentgenerator 314 may thus be used for influencing the amplification of thecircuit.

It should be noted that the figures only show preferred embodiments.Within the scope of the invention is of course the possibility that forexample the polarity of the circuit may be the opposite. With referenceto FIG. 2, for example the photo-diode 301 could be reversed and V1could be a negative voltage. The control unit 314 would in this casecontrol a current out from the first input 304.

FIG. 3 shows a further embodiment of the invention. The correspondingparts as in FIG. 2 have the same reference signs as in FIG. 2. Theseparts will therefore not be described more closely in connection withFIG. 3. According to FIG. 3, the control unit constitutes a variableamplification-controlling voltage unit 316 connected to a firstresistance 318 which in its turn is connected to the first input 304. Asecond diode unit 320 is arranged for preventing an incorrect currentdirection, i.e. in this case a current out from the first input 304. Afilter member 322, in this case a capacitor, is arranged to filter outpossible disturbances which are superposed on theamplification-controlling voltage.

A second resistance 324 is connected to the anode of the photodiode 301for conducting away a direct current. According to the shown embodiment,the amplifier unit 302 is of the kind where a low current into the firstinput 304 means a high amplification. In order to ensure that a certaincurrent is always present at the first input 304, a third resistance 326is arranged between the bias voltage V1 and the first input 304. Thisthird resistance 326 thus limits the amplification of the circuit.

As has been mentioned above, it may be interesting to measure the photocurrent through the photo-diode 301. The photo current is proportionalto the voltage over the second resistance 324. However, it may beunsuitable to measure this voltage since such a measurement could leadto disturbances in a sensitive part of the circuit. In order to avoidthis problem, a fourth resistance 330 is connected to the anode of thephoto-diode 301. This fourth resistance is suitably a resistance with ahigh resistance. A measurement device 332 may thus be connected to thisfourth resistance 330.

334 symbolises a circuit which follows after the amplifier unit 302.This circuit 334 may comprise an amplifier. The hatched line 336symbolises that a feedback from this circuit 334 may be arranged forinfluencing the variable voltage which is symbolised by the unit 316.

FIG. 4 shows schematically an optical communication system according tothe invention. Such a communication system comprises a firsttransmitter-receiver unit 340. This unit comprises a transmitter 342 anda receiver 344. A second transmitter-receiver unit is designated by 346.This second transmitter-receiver unit 346 comprises a receiver 348 and atransmitter 350. The communication system comprises a first 352 and asecond 354 optical fibre. According to the invention, at least one ofsaid transmitter-receiver units 340, 346 comprises an amplifier circuitaccording to the invention. Suitably both transmitter-receiver units340, 346 comprises such an amplifier circuit.

The invention also concerns a method of controlling the amplification inan optical input stage of the kind which has been described above, i.e.an amplifier circuit which comprises a light sensitive member 301arranged to receive an optical input signal and to deliver an electricsignal in response to the received optical signal, and an amplifier unit302 with a first input 304 arranged to receive the electric signal fromthe light sensitive member 301, wherein the amplifier unit 302 isarranged to influence the amplification of the electric signal and todeliver an amplified output signal. The amplifier unit 302 is of thekind which does not have any further input specially intended forcontrolling the amplification of the amplifier unit. According to themethod, a filter unit 310 is arranged for preventing a possible directcurrent in an electric signal from the light sensitive member 301 fromreaching the first input 304 of the amplifier unit 302. Furthermore, acontrol unit 314 is connected to the first input 304 for controlling thepower of the electric signal at this first input 304. Thereby theamplification of the input stage is influenced.

Advantageous embodiments of the method are clear from the claims 15 and16. Further advantageous embodiments of the method are clear from theabove-described function of the amplifier circuit.

The invention is not limited to the described embodiments but may bevaried and modified within the scope of the following claims.

1. An amplifier circuit for receiving an optical signal, which amplifiercircuit comprises: a light sensitive member arranged to receive anoptical input signal and to deliver an electric signal in response tothe received optical signal, an amplifier unit with a first inputarranged to receive the electric signal from the light sensitive member,wherein the amplifier unit is arranged to influence the amplification ofthe electric signal and to deliver an amplified output signal via afirst output, wherein the amplifier unit does not have any further inputspecially intended for controlling the amplification of the amplifierunit, characterised by a filter unit arranged to prevent a possibledirect current in the electric signal from the light sensitive memberfrom reaching said first input, wherein said light sensitive member isarranged such that a possible direct current from the light sensitivemember would be directed into said amplifier unit via the first input ifsaid filter unit were not arranged to prevent such a direct current, ana control unit connected to said first input and arranged to control thepower of an electric current in a direction in via the first input forthereby influencing the power of said output signal.
 2. An amplifiercircuit according to claim 1, wherein the control unit is arranged as acontrollable current generator in that a variableamplification-controlling voltage unit is connected to a firstresistance connected to said first input.
 3. An amplifier circuitaccording to claim 1, wherein said light sensitive member comprises afirst diode unit with a first connection connected to a bias voltage anda second connection connected to said filter unit, which filter unit isconnected to the first input.
 4. An amplifier circuit according to claim3, wherein the first connection constitutes the cathode side of thefirst diode unit and the second connection constitutes the anode side ofthe first diode unit.
 5. An amplifier circuit according to claim 3,comprising a second resistance connected to said second connection forconducting away a possible direct current through the first diode unit.6. An amplifier circuit according to claim 1, comprising anamplification limiting unit connected to said first input.
 7. Anamplifier circuit according to claim 6, wherein the amplificationlimiting unit comprises a third resistance connected between a voltagesource and said first input.
 8. An amplifier circuit according to claim1, wherein said amplifier unit comprises an internal amplificationcontrolling unit, for example arranged for reducing the amplification attoo high currents.
 9. An amplifier circuit according to claim 1,comprising a second diode unit connected to said control unit forpreventing incorrect current direction to/from the first input.
 10. Anamplifier circuit according to claim 1, comprising a filter memberconnected to said control unit for filtering out possible disturbancesof the electric signal which is controlled with the control unit.
 11. Anamplifier circuit according to claim 1, comprising a fourth resistancewith a first side connected to said light sensitive member and a secondside arranged to be connectable to a measuring unit for measuring thecurrent through the light sensitive member.
 12. An optical communicationsystem comprising a first transmitter-receiver unit, a secondtransmitter-receiver unit and at least one optical conduction memberwhich connects the first and the second transmitter-receiver units,wherein at least one of said transmitter-receiver units comprises anamplifier circuit according to claim
 1. 13. A method for controlling theamplification in an optical input stage which comprises a lightsensitive member arranged to receive an optical input signal and todeliver an electric signal in response to the received optical signal,an amplifier unit with a first input arranged to receive the electricsignal from the light sensitive member, wherein the amplifier unit isarranged to influence the amplification of the electric signal and todeliver an amplified output signal via a first output , wherein theamplifier unit does not have any further input specially intended forcontrolling the amplification of the amplifier unit , which methodcomprises the following steps: that a filter unit is arranged to preventa possible direct current in the electric signal from the lightsensitive member from reaching said first input, wherein said lightsensitive member is arranged such that a possible direct current fromthe light sensitive member would be directed into said amplifier unitvia the first input if said filter unit were not arranged to preventsuch a direct current, and that a control unit is connected to saidfirst input and arranged to control the power of an electric current ina direction in via the first input for thereby influencing the power ofsaid output signal.
 14. A method according to claim 13, wherein thecontrol unit is arranged as a controllable current generator in that avariable amplification controlling voltage is applied over a firstresistance connected to said first input.
 15. A method according toclaim 14, wherein said light sensitive member (301) is arranged suchthat a possible direct current from the light sensitive member (301)would be directed into said amplifier unit (302) via the first input(304) if said filter unit (310) were not arranged to prevent such adirect current, wherein the control unit (314) is arranged to controlthe power of an electric current in a direction in via the first input(304).
 16. A method according to claim 14 or 15, wherein the controlunit (314) is arranged as a controllable current generator in that avariable amplification controlling voltage (316) is applied over a firstresistance (318) connected to said first input (304).